Project Abstract Adolescents in sub-Saharan Africa make the transition to adulthood facing considerable risk of HIV infection; this risk is particularly high for young women. Understanding social processes that produce HIV risk and protect young people is critical to developing the next generation of youth HIV prevention programs. Looking beyond individual characteristics and behaviors, social structural factors (socioeconomic status, educational opportunities, gender, AIDS orphanhood, public policies) and timing of adolescent and young adult social role transitions (leaving school, initiation of sexual behaviors, migration and leaving home, marriage formation and dissolution) are critical drivers of youth HIV acquisition and other reproductive outcomes. Yet, these findings raise new questions: how do the interrelationships among social transitions and the timing, ordering and tempo of these transitions influence HIV risk behaviors, and, ultimately, HIV infection? How do social structural factors influence HIV infection? How do social transitions mediate HIV risk? To answer these questions, new research is needed which applies innovative quantitative and qualitative methods to high quality longitudinal data. Over the past six years, the Rakai Youth Project has used new qualitative data and existing quantitative and longitudinal data from the Rakai Community Cohort Study (RCCS) to successfully define a continuum of social and proximate determinants for HIV acquisition among youth ages 15?24 from 1994? 2013. Building on this work, Structural and Social Transitions among Adolescents and young adults in Rakai (SSTAR) will investigate the influence of social structural determinants on transitions from adolescence to adulthood using innovative statistical and qualitative research methodologies. SSTAR will define risk factors for and trends over time in key social transitions (sexual initiation, school leaving, marital formation and dissolution, migration, initiation of childbearing); interrelationships among transitions (ordering, timing, tempo); and consequences (HIV risk behaviors, acquisition) among adolescents and young adults using mixed methods. The proposed project will examine the influence on HIV acquisition of social structural determinants (access to schooling, SES, orphanhood, household structure, gender, government policy, HIV programs) directly and as mediated by social transitions, using the RCCS. Finally, SSTAR will explore the influence of social determinants and social transitions on HIV acquisition in: 1-high HIV incidence hotspot communities, 2- intermediate HIV incidence trading and transport hub communities, and 3- low HIV incidence settled, rural communities.